The invention relates to a structure for damping impact by mechanical deformation, the said structure being of elongate shape in an impact direction and of substantially constant running cross section.
By running section is meant the section over the essential part of the length of the structure for damping impact.
The invention applies in particular to railway vehicles.
The specifications of vehicles carrying passengers, and especially railway vehicles, require the presence of such structures for damping impact which are capable, by virtue of planned mechanical deformation, of absorbing the energy in the event of an accident to save human lives and safeguard equipment.
The energy absorption capacity requirements for such structures are higher and higher at a time when simultaneous efforts are being made to reduce the weights of the vehicles.
Such structures for damping impact must therefore be light and resistant to static forces. Furthermore, in the event of an impact, they must deform by being crushed in the manner of a "concertina", essentially in the impact direction without transverse displacement or corkscrewing.
Document EP-B-621,416 describes such a structure for damping impact which is formed by longitudinal metal plates, welded together. The running cross section of the structure, which is constant in the impact direction, comprises a rectangle whose angles are connected by diagonals which form an X within the rectangle.
In the event of impact, deformation will begin randomly at either of the two longitudinal ends of the structure, which may result in unstable deformation and hence relatively low energy absorption.
Furthermore, it is established that, in the event of an impact, the longitudinal force borne in the impact direction by such a structure first reaches a peak threshold value above which crushing begins, then the longitudinal force declines sharply as a result of which the energy absorbed by the damping structure, equal to the area of the force/deformation curve, is relatively low.
It is therefore desirable to be able to limit the reduction in the longitudinal force borne by the impact damping structure in the course of its deformation.
However, this longitudinal force, corresponding to the acceleration experienced by the passengers in the vehicle, must not be too high for passenger safety.
Finally, it is established that the deformation of such a structure is limited by the accumulation of material taking place at the center of the X of the cross section during the crushing of the structure. Hence, the energy absorption capacity of such a structure is again limited.